Deep in the lung there are millions of alveoli. These grape-shaped cavities are where gas-exchange occurs. The air flow itself can be viewed as an incompressible Newtonian fluid. This 'fluid' can carry many differently sized particles. Fine particles (such as pollutants and inhaled therapeutic drugs) can penetrate into the alveolar cavities. Although these particles have little inertia, low diffusivity and are approximately neutrally buoyant, their motion in this region of the lung is kinematically irreversible. Some of this complex behaviour has been modelled physically, and can manifest from a basic recirculation pattern. The image on the right shows a recirculation region within a single alveolus (the bar represents 100 microns). This recirculation has been induced by half a breath - further breathing has been shown to disrupt such regular patterns into chaos. Here at Surrey, we collaborate with members of Harvard University to understand how this can happen. In particular, we are interested in the differences between adult and postnatal lungs as, comparatively, the latter do not have many alveoli at the early stages of childhood.